Adjustment of a metrology apparatus or a measurement thereby based on a characteristic of a target measured

ABSTRACT

A method of adjusting a metrology apparatus, the method including: spatially dividing an intensity distribution of a pupil plane of the metrology apparatus into a plurality of pixels; and reducing an effect of a structural asymmetry in a target on a measurement by the metrology apparatus on the target, by adjusting intensities of the plurality of pixels.

This application is a continuation of U.S. patent application Ser. No.16/061,209, filed on Jun. 11, 2018, now allowed, which is the U.S.national phase entry of PCT/EP2016/079105, filed on Nov. 29, 2016, whichclaims the benefit of priority of U.S. Patent Application No.62/268,974, filed on Dec. 17, 2015, each of the foregoing applicationsis incorporated herein in its entirety by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates to adjustment of metrology in electronicdevice processing based on a characteristic of the target measured.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes (SEMs), which are often used tomeasure critical dimension (CD), and specialized tools to measureoverlay (the accuracy of alignment of two layers in a device) of thelithographic apparatus. Recently, various forms of scatterometers havebeen developed for use in the lithographic field. These devices direct abeam of radiation onto a target and measure one or more properties ofthe scattered radiation—e.g., intensity at a single angle of reflectionas a function of wavelength; intensity at one or more wavelengths as afunction of reflected angle; or polarization as a function of reflectedangle—to obtain a “spectrum” from which a property of interest of thetarget can be determined. Determination of the property of interest maybe performed by various techniques: e.g., reconstruction of the targetstructure by iterative approaches such as rigorous coupled wave analysis(RWCA) or finite element methods; library searches; and principalcomponent analysis.

The targets used by conventional scatterometers are relatively large(e.g., 40 μm by 40 μm) gratings and the measurement beam generates aspot that is smaller than the grating (i.e., the grating isunderfilled). This simplifies mathematical reconstruction of the targetas it can be regarded as infinite. However, in order to reduce the sizeof the targets, e.g., to 10 μm by 10 μm or less, so they can bepositioned in amongst product features rather than in the scribe lines,metrology has been proposed in which the grating is made smaller thanthe measurement spot (i.e., the grating is overfilled). Typically suchtargets are measured using dark-field scatterometry in which the zerothorder of diffraction (corresponding to a specular reflection) isblocked, and only higher orders processed.

Diffraction-based overlay (DBO) using dark-field detection of the firstorder diffraction orders enables overlay measurements on smallertargets. These targets can be smaller than the illumination spot and maybe surrounded by product structures on a wafer. Multiple targets can bemeasured in one image. In the known metrology technique, overlaymeasurement results are obtained by measuring the target twice undercertain conditions, while either rotating the target or changing theillumination mode or imaging mode to obtain separately the −1^(st) andthe +1^(st) diffraction order intensities. Comparing these intensitiesfor a given grating provides a measurement of asymmetry in the grating.

One approach to improve metrology accuracy is better control at thedesign stage of overlay metrology targets compatible to knownillumination mode and other process constraints so that structuralasymmetry in the target is minimized by design. This disclosureaddresses an alternative approach where even if structural asymmetryexists in the target, illumination is controlled to negate the effect ofasymmetry in overlay measurement. In other words, the measurementaccuracy is improved by utilizing the flexibility in reconfiguring theillumination according to embodiments of the present disclosure ratherthan changing the target design.

SUMMARY

Disclosed herein is a method of adjusting a metrology apparatus, themethod comprising: spatially dividing an intensity distribution in apupil plane of the metrology apparatus into a plurality of pixels;reducing an effect of a structural asymmetry in a target on ameasurement by the metrology apparatus on the target, by adjusting,using a computer, intensities of the plurality of pixels.

Disclosed herein is a method of adjusting a metrology apparatus, themethod comprising: adjusting a parameter of the metrology apparatus orof a measurement by the metrology apparatus on a target, based on acharacteristic of the target.

Disclosed herein is a method comprising: setting a parameter of ametrology apparatus or of a measurement by the metrology apparatus on atarget to a value adjusted based on a characteristic of the target;measuring the target with the metrology apparatus.

Disclosed herein is a method comprising: setting a characteristic of atarget to a value based on which a parameter of a metrology apparatus orof a measurement by the metrology apparatus on the target is adjusted;fabricating the target on a substrate.

Disclosed herein is a computer program product comprising anon-transitory computer-readable medium having instructions thereon, theinstructions when executed by a computer implementing any of the abovemethods.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present disclosure willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the disclosure inconjunction with the accompanying figures, wherein:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe present disclosure;

FIG. 2A shows a schematic diagram of a dark-field scatterometer for usein measuring targets according to embodiments of the disclosure using afirst pair of illumination apertures;

FIG. 2B shows a detail of diffraction spectrum of a target grating for agiven direction of illumination;

FIG. 3A depicts a cross sectional view of a grating;

FIG. 3B, FIG. 3C and FIG. 3D depict the principle of overlay measurementbetween two vertical layers of gratings using diffraction-basedscatterometry;

FIG. 4A, FIG. 4B, FIG. 4C, FIG. 4D, FIG. 4E and FIG. 4F each showvarious stages of illumination shaping by apertures and pupil filters,according to an embodiment of the present disclosure;

FIG. 5 shows a target with a multilayer memory device structure used toevaluate the efficacy of the illumination optimization according to anembodiment;

FIG. 6A and FIG. 6B show examples of lithography process-inducedasymmetry in target structure;

FIG. 7 shows a table (Table I) listing results of the overlaysensitivity calculation for various illumination polarizations;

FIGS. 8-10 show change in illumination shape according to theoptimization algorithm disclosed herein;

FIGS. 11-14 show various mathematical functions plotted against thenumber of iterations used in the optimization algorithm disclosedherein;

FIG. 15 schematically shows a flow for a method according to anembodiment;

FIG. 16 schematically shows a flow for a method according to anembodiment.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporatethe features of this disclosure. The disclosed embodiment(s) merelyexemplify the inventive concepts. The scope of the disclosure is notlimited to the disclosed embodiment(s). The disclosure is defined by theclaims appended hereto. The embodiment(s) described, and references inthe specification to “one embodiment”, “an embodiment”, “an exampleembodiment”, etc., indicate that the embodiment(s) described may includea particular feature, structure, or characteristic, but every embodimentmay not necessarily include the particular feature, structure, orcharacteristic. Moreover, such phrases are not necessarily referring tothe same embodiment. Further, when a particular feature, structure, orcharacteristic is described in connection with an embodiment, it isunderstood that it is within the knowledge of one skilled in the art toaffect such feature, structure, or characteristic in connection withother embodiments whether or not explicitly described.

Embodiments of the disclosure may be implemented in hardware, firmware,software, or any combination thereof. Embodiments of the disclosure mayalso be implemented as instructions stored on a machine-readable medium,which may be read and executed by one or more processors. Amachine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputing device). For example, a machine-readable medium may includeread only memory (ROM); random access memory (RAM); magnetic diskstorage media; optical storage media; flash memory devices; electrical,optical, acoustical or other forms of propagated signals (e.g., carrierwaves, infrared signals, digital signals, etc.), and others. Further,firmware, software, routines, instructions may be described herein asperforming certain actions. However, it should be appreciated that suchdescriptions are merely for convenience and that such actions in factresult from computing devices, processors, controllers, or other devicesexecuting the firmware, software, routines, instructions, etc.

Before describing such embodiments in more detail, however, it isinstructive to present an example environment in which embodiments ofthe present disclosure may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a patterningdevice support or support structure (e.g., a mask table) MT constructedto support a patterning device (e.g., a mask) MA and connected to afirst positioner PM configured to accurately position the patterningdevice in accordance with certain parameters; a substrate table (e.g., awafer table) WT constructed to hold a substrate (e.g., a resist coatedwafer) W and connected to a second positioner PW configured toaccurately position the substrate in accordance with certain parameters;and a projection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., including one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types.

An example of a programmable mirror array employs a matrix arrangementof small mirrors, each of which can be individually tilted so as toreflect an incoming radiation beam in different directions. The tiltedmirrors impart a pattern in a radiation beam, which is reflected by themirror matrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. The intensity distribution in the pupilplane is referred to as the “illumination pupil” elsewhere in thespecification. In addition, the illuminator IL may include various othercomponents, such as an integrator IN and a condenser CO. The illuminatormay be used to condition the radiation beam, to have a desireduniformity and intensity distribution in its cross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan.

In general, movement of the patterning device support (e.g., mask table)MT may be realized with the aid of a long-stroke module (coarsepositioning) and a short-stroke module (fine positioning), which formpart of the first positioner PM. Similarly, movement of the substratetable WT may be realized using a long-stroke module and a short-strokemodule, which form part of the second positioner PW. In the case of astepper (as opposed to a scanner) the patterning device support (e.g.,mask table) MT may be connected to a short-stroke actuator only, or maybe fixed.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. The alignment system, whichdetects the alignment markers is described further below.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support (e.g., mask table) MT andthe substrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e., a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed. In step mode, the maximum size of theexposure field limits the size of the target portion C imaged in asingle static exposure.

2. In scan mode, the patterning device support (e.g., mask table) MT andthe substrate table WT are scanned synchronously while a patternimparted to the radiation beam is projected onto a target portion C(i.e., a single dynamic exposure). The velocity and direction of thesubstrate table WT relative to the patterning device support (e.g., masktable) MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the patterning device support (e.g., mask table) MTis kept essentially stationary holding a programmable patterning device,and the substrate table WT is moved or scanned while a pattern impartedto the radiation beam is projected onto a target portion C. In thismode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after eachmovement of the substrate table WT or in between successive radiationpulses during a scan. This mode of operation can be readily applied tomaskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A dark field metrology apparatus suitable for use in embodiments of thedisclosure is shown in FIG. 2A, although the disclosure is not limitedto dark-field scatterometry only. In the example shown in FIG. 2A, atarget grating T is illuminated and rays diffracted from the grating arecollected for measurement. This is illustrated in more detail in FIG.2B. The dark field metrology apparatus may be a stand-alone device orincorporated in either the lithographic apparatus LA shown in FIG. 1,e.g., at the measurement station, or a lithographic cell. An opticalaxis, which has several branches throughout the apparatus, isrepresented by a dotted line O. In this apparatus, light emitted bysource 11 (e.g., a xenon lamp) is directed onto substrate W via a beamsplitter 15 by an optical system comprising lenses 12, 14 and objectivelens 16. These lenses are arranged in a double sequence of a 4Farrangement. A different lens arrangement can be used, provided that itstill provides a substrate image onto a detector, and simultaneouslyallows for access of an intermediate pupil-plane for spatial-frequencyfiltering. Therefore, the angular range at which the radiation isincident on the substrate can be selected by defining a spatialintensity distribution in a plane that presents the spatial spectrum ofthe substrate plane, here referred to as a (conjugate) pupil plane. Inparticular, this can be done by inserting an aperture plate 13 ofsuitable form between lenses 12 and 14, in a plane which is aback-projected image of the objective lens pupil plane. In the exampleillustrated, aperture plate 13 has different forms, labeled 13N and 13S,allowing different illumination modes to be selected. The illuminationsystem in the present examples forms an off-axis illumination mode. Inthe first illumination mode, aperture plate 13N provides off-axis from adirection designated, for the sake of description only, as ‘north’. In asecond illumination mode, aperture plate 13S is used to provide similarillumination, but from an opposite direction, labeled ‘south’. Othermodes of illumination are possible by using different apertures. Therest of the pupil plane is desirably dark as any unnecessary lightoutside the desired illumination mode will interfere with the desiredmeasurement signals.

As shown in FIG. 2B, target grating T is placed with substrate W normalto the optical axis O of objective lens 16. A ray of illumination Iimpinging on grating T from an angle off the axis O gives rise to azeroth order component of illumination (solid line 0) and two firstorder components of illumination (dot-chain line +1 and double dot-chainline −1). It should be remembered that with an overfilled small targetgrating, these rays of illumination are just one of many parallel rayscovering the area of the substrate including metrology target grating Tand other features. Since the aperture in plate 13 has a finite width(necessary to admit a useful quantity of light, the incident rays I willin fact occupy a range of angles, and the diffracted rays of 0 and +1/−1order will be spread out somewhat. According to the point spreadfunction of a small target, each order +1 and −1 will be further spreadover a range of angles, not a single ideal ray as shown. Note that thegrating pitches and illumination angles can be designed or adjusted sothat the first order rays entering the objective lens are closelyaligned with the central optical axis. The rays illustrated in FIGS. 2Aand 2B are shown somewhat off axis, purely to enable them to be moreeasily distinguished in the diagram.

At least the 0 and +1 or −1 orders diffracted by the target on substrateW are collected by objective lens 16 and directed back through beamsplitter 15. Returning to FIG. 2A, both the first and secondillumination modes are illustrated, by designating diametricallyopposite apertures labeled as north (N) and south (S). In otherembodiments, East (E) and West (W) labels may be used depending on theposition of the apertures. When the incident ray I is from the northside of the optical axis, that is when the first illumination mode isapplied using aperture plate 13N, the +1 diffracted rays, which arelabeled +1(N), enter the objective lens 16. In contrast, when the secondillumination mode is applied using aperture plate 13S the −1 diffractedrays (labeled −1(S)) are the ones which enter the lens 16.

A second beam splitter 17 divides the diffracted beams into twomeasurement branches. In a first measurement branch, optical system 18forms a diffraction spectrum (pupil plane image) of the target on firstsensor 19 (e.g. a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements of thefirst order beam.

In the second measurement branch, optical system 20, 22 forms an imageof the target on the substrate W on sensor 23 (e.g. a CCD or CMOSsensor). In the second measurement branch, an aperture stop 21 isprovided in a plane that is conjugate to the pupil-plane. Aperture stop21 functions to block the zeroth order diffracted beam so that the imageof the target formed on sensor 23 is formed only from the −1 or +1 firstorder beam. The images captured by sensors 19 and 23 are output to imageprocessor and controller PU, the function of which will depend on theparticular type of measurements being performed. Note that the term‘image’ is used here in a broad sense. An image of the grating lines assuch will not be formed, if only one of the −1 and +1 orders is present.

The particular forms of aperture plate 13 and field stop 21 shown inFIG. 2A are purely examples. In another embodiment of the disclosure,on-axis illumination of the targets is used and an aperture stop with anoff-axis aperture is used to pass substantially only one first order ofdiffracted light to the sensor. In yet other embodiments, 2^(nd), 3^(rd)and higher order beams (not shown in FIG. 2B) can be used inmeasurements, instead of or in addition to the first order beams.

In order to make the illumination adaptable to these different types ofmeasurement, the aperture plate 13 may comprise a number of aperturepatterns formed around a disc, which rotates to bring a desired patterninto place. Alternatively or in addition, a set of plates 13 could beprovided and swapped, to achieve the same effect. A programmableillumination device such as a deformable mirror array or transmissivespatial sight modulator (SLM) can be used also. Moving mirrors or prismscan be used as another way to adjust the illumination mode.

As explained in relation to aperture plate 13, the selection ofdiffraction orders for imaging can alternatively be achieved by alteringthe pupil-stop 21, or by substituting a pupil-stop having a differentpattern, or by replacing the fixed field stop with a programmable SLM.In that case the illumination side of the measurement optical system canremain constant, while it is the imaging side that has first and secondmodes. In the present disclosure, therefore, there are effectively threetypes of measurement method, each with its own advantages anddisadvantages. In one method, the illumination mode is changed tomeasure the different orders. In another method, the imaging mode ischanged. In a third method, the illumination and imaging modes remainunchanged, but the target is rotated through 180 degrees. In each casethe desired effect is the same, namely to select first and secondportions of the non-zero order diffracted radiation which aresymmetrically opposite one another in the diffraction spectrum of thetarget. In principle, the desired selection of orders could be obtainedby a combination of changing the illumination modes and the imagingmodes simultaneously, but that is likely to bring disadvantages for noadvantage, so it will not be discussed further.

While the optical system used for imaging in the present examples has awide entrance pupil which is restricted by the field stop 21, in otherembodiments or applications the entrance pupil size of the imagingsystem itself may be small enough to restrict to the desired order, andthus serve also as the field stop.

Typically, a target grating will be aligned with its grating linesrunning either north-south or east-west. That is to say, a grating willbe aligned in the X direction or the Y direction of the substrate W.Note that aperture plate 13N or 13S can only be used to measure gratingsoriented in one direction (X or Y depending on the set-up). Formeasurement of an orthogonal grating, rotation of the target through 90°and 270° might be implemented. As mentioned already, the off-axisapertures could be provided in field stop 21 instead of in illuminationaperture plate 13. In that case, the illumination would be on axis.

Additional aperture plates can be used to combine the illumination modesdescribed above. Provided that cross-talk between these differentdiffraction signals is not too great, measurements of both X and Ygratings can be performed without changing the illumination mode.

As mentioned before, metrology targets often take the form of one ormore gratings. The gratings can be placed in the scribelines or in theproduct area. If the gratings are located in product areas, productfeatures may also be visible in the periphery of an image field. Imageprocessor and controller PU processes these images using patternrecognition to identify the separate images if a plurality of gratingsis used.

While the target structures described above are metrology targetsspecifically designed and formed for the purposes of measurement, inother embodiments, properties may be measured on targets which arefunctional parts of devices formed on the substrate. Many devices haveregular, grating-like structures. The terms ‘target grating’ and ‘targetstructure’ as used herein do not require that the structure has beenprovided specifically for the measurement being performed.

In association with the physical grating structures of the targets asrealized on substrates and patterning devices, an embodiment may includea computer program containing one or more sequences of machine-readableinstructions describing a methods of producing targets on a substrate,measuring targets on a substrate and/or analyzing measurements to obtaininformation about a lithographic process. This computer program may beexecuted for example within unit PU in the apparatus of FIG. 2 and/or acontrol unit in the lithography apparatus. There may also be provided adata storage medium (e.g., semiconductor memory, magnetic or opticaldisk) having such a computer program stored therein. Where an existingmetrology apparatus, for example of the type shown in FIG. 2A, isalready in production and/or in use, the disclosure can be implementedby the provision of updated computer program products for causing aprocessor to perform the methods described herein and so calculateexposure dose. The program may optionally be arranged to control theoptical system, substrate support and the like to perform the steps formeasurement of a suitable plurality of target structures.

FIG. 3A illustrates the cross sectional view a standard single-layergrating pattern. In FIG. 3A, a limited section of only five periods ofthe grating is shown. The grating period is Pd, horizontal line width ofindividual grating lines is w1, and vertical thickness of each gratingline is t. In the full grating, the pattern may repeat in the vertical(z axis) and horizontal (x axis) or lateral (y axis) directions. Thegrating pattern in FIG. 3A may comprise, for example, a chrome patternon a reticle. The parameters Pd, w1, and t may be used to describeaspects of the grating, along with other parameters.

While grating sections may be on a same device layer and used forsame-layer alignment and/or image stitching purposes, FIGS. 3B-D showhow grating-based metrology structures can be used to measure overlayand/or alignment between two different layers in an electronic device. Afirst grating G1 is on layer one, and a second grating G2 is on layertwo, where the two layers mimic device layers in an actual device to bemanufactured (also referred to as “product”). The device may have afirst pattern in layer one and a second pattern in layer two, and theoverlay and/or alignment between those two patterns are of interest.FIG. 3B shows overlay with a bias in the negative direction (along thehorizontal x axis), and FIG. 3D shows overlay with a bias in thepositive direction (along the x axis). The x axis is also referred to asthe direction of overlay. FIG. 3C shows the case where there is nooverlay between G1 and G2, as they are aligned (along the vertical zdirection) perfectly. Persons skilled in the art would appreciate that adesigner may intentionally add positive or negative bias along the xdirection for measurement and image matching purposes, and measure theeffect of bias by measuring the variation of intensity (I) of thediffracted beams. Here, only the zero-order, +1 order and −1 orderdiffracted beams are shown for simplicity, and higher order beams arenot shown.

In a diffraction-based metrology apparatus, such as YieldStar by ASML,illumination intensity distribution can be adjusted by adding a SpatialLight Modulator (SLM) in the illumination. SLMs imposes some form ofspatially varying intensity and/or modulation on a beam of radiation.SLMs may comprise physical mechanisms such as programmable mirrorarrays, programmable LCD panels etc., or a computer representationthereof. Addition of an SLM enables a free-form illumination. Theintensity distribution in the pupil plane of the illumination isrepresented in the simulation domain by a spatial array of pixels, whereeach pixel or a group of pixels can be independently adjusted to achievevarious configurations of the illumination depending on desiredlithographic imaging performance. Addition of the SLM helps in reductionof stray light in the system, and possibly reduces the effect of markasymmetry on the overlay measurement. The term “mark” is used here toinclude a pattern in a design layout which is used as a metrologytarget. The mark may be part of the circuit, or a special metrology markplaced in a scribe line of the wafer. This disclosure assumes the markis not altered, but the illumination configuration is altered to negatethe effect of process-induced structural asymmetry in the mark. Byproper selection of pixels in the illumination pupil, it is possible toreduce the effect of mark asymmetry without impacting the contrast (i.e.detectability) of the mark.

Particularly, this disclosure focuses on the feasibility of adjusting aparameter of a metrology apparatus (e.g., controlling illumination) toimprove the quality of the measurement (e.g., overlay detectability andmeasurement accuracy). It is possible to optimize both the illumination(i.e. the “source”) and the overlay metrology target (i.e. the “mark”)to improve measurement accuracy (“Source-Mark Optimization”). Here theterm “source” broadly includes both the radiation source andillumination adjustment optics included in a radiation system. Thepresent disclosure provides an illumination optimization algorithm,including free-form illumination optimization, for a given overlaymetrology target. Specifically, the disclosure describes selection ofillumination points to, among other things, eliminate stray lights,generate sufficient strength of optical signal for detection andanalysis, provide improved contrast of the mark, and, decreasesensitivity to process variation. The source is not the only parameterthat can be adjusted based on a characteristic of the target. Any otherparameter of the metrology apparatus or the measurement may be adjusted.

There are areas of the illumination pupil that do not produce ±1 orderinformation. Those areas only add zeroth order. Although the zerothorder is filtered, the area of the pupil does produce stray light byreflections in the optics “bleeding” into the ±1 order area. Usingsimple geometry, these 0 order areas of the illumination are eliminated.The location of this 0 order area is pitch and wavelength dependent.Hence a spatial light modulator (SLM) or a similar device is needed tochange the illumination.

Imaging conditions in the lithographic apparatus may be made similar tothat of an existing diffraction-based metrology apparatus, such asYieldStar, manufactured by ASML. In a non-limiting exampleconfiguration, the illumination may have a numerical aperture (NA) of0.95. FIG. 4A shows an illumination pupil with a standard quadrantaperture in a lithographic apparatus. In a metrology apparatus, usuallythere is no quadrant aperture. A pupil of the objective is imaged viafour wedges to re-direct zeroth order signal away from the sensor. Incontrast, lithographic apparatuses use pupil filters of various shapes.One of the advantages of the present disclosure is that whenillumination optimization is performed in the simulation domain, anyshape of aperture and pupil filter can be assumed to mimic the actualconfiguration of the illumination pupil of the lithographic apparatus.

FIG. 4B shows an example illumination shape at the entrance pupil of theobjective lens in a metrology apparatus. Zeroth order illuminationpasses through the upper right and lower left quadrants of the pupil asshown in FIG. 4B. By using a pupil filter such as the one shown in FIG.4C, it is possible to block most of the zeroth order illumination, whileletting the +1 and −1 order of the illumination pass through to theobjective lens. FIG. 4D shows the shape of the illumination at the exitpupil of the objective lens. FIG. 4E shows the unutilized areas of theillumination. FIG. 4F shows a better choice of a illumination pupilshape (compared to the standard quadrant aperture shown in FIG. 4A)where exit pupil of the objective would be optimized to produce betterresults to counter the negative effect of structural asymmetry in thetarget, which is a major goal of this disclosure.

To achieve decreased sensitivity to lithography process variation,simulations are performed with various target designs mimicking actualdevices. Example devices include, but are not limited to, Dynamic RandomAccess Memory (DRAM) buried word line (bWL) layer, DRAM bit line (BL)layer, Logic metal layer (M1), and DRAM storage node (SN) layer.

In an embodiment, using an exemplary circuit pattern for a DRAM storagenode (SN), it was determined which pupil pixels would decreasesensitivity to the asymmetry induced by process variation. The metrologytarget (“mark”) was made to replicate the DRAM SN layer pattern, asshown in FIG. 5, because such a pattern is known to have overlaysensitivity to process-induced structural asymmetry. In the structureshown in FIG. 5, grating pitch is 500 nm, critical dimension of the toplayer is 250 nm. The illumination wavelength is 550 nm. An illuminationpupil shape shown in FIG. 4F is used as the starting condition forillumination optimization. In the example device structure 500, the topmetal layer 502 is patterned using the resist layer 504. Ananti-reflection coating layer 506, a carbon layer 508, an inter-layerdielectric (ILD) separate the top layer from the buried storage nodelayer 514 made of tungsten. The substrate layer 512 is made of silicon.Persons skilled in the art would appreciate that the scope of thedisclosure is not limited by the materials used in the device structure.

In addition to decreasing sensitivity to process variation, the markneeds to have sufficient diffraction efficiency (DE) and stacksensitivity (SS), so sufficient illumination pupil pixels are needed toreconfigure the illumination shape to study the effect of a particularasymmetry. DE is the target design parameter that represents the averageintensity and SS is related to DE. DE is expressed as:DE=¼[(I _(p_pb) +I _(m_pb))+(I _(p_nb) +I _(m_nb))]  (Eq. 1)

In this equation, I indicates intensity, p in the suffix indicates +1order diffraction component, m in the suffix indicates −1 orderdiffraction component, pb in the suffix indicates positive bias, nb inthe suffix indicates negative bias. For example, I_(p_pb) means theintensity of the +1 order diffraction component with positive bias.

The present approach first examines gradients of intensity to verifythat certain illumination configuration can minimize the effect ofasymmetry. An asymmetry parameter q is defined. The following equationsestablish how the parameter q is related to overlay.

The overlay, OV, is defined as,

$\begin{matrix}{{OV} = \frac{A}{K}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$where A=½[(I _(p_pb) −I _(m_pb))+(I _(p_nb) −I _(m_nb))]  (Eq. 3)

Here, A is asymmetry response, and, K is a mathematical constantrepresenting overlay error, where

$\begin{matrix}{K = {\frac{1}{2d}\left\lbrack {\left( {I_{p\;\_\;{pb}} - I_{m\;\_\;{pb}}} \right) - \left( {I_{p\;\_\;{nb}} - I_{m\;\_\;{nb}}} \right)} \right\rbrack}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

In this equation, d is the overlay bias. K represents contrast in thetarget image due to overlay error. Overlay bias d can be in the positiveor negative direction, as shown in FIGS. 3B and 3D respectively. StackSensitivity (SS) is defined as K/DE, and is a key performance indicator(KPI) for target design. One of the goals of target design is tomaximize SS. Overlay error due to asymmetry goes to zero when A goes tozero.

The algorithm used to optimize the illumination pixel by pixel for aknown asymmetry in the target is based on figuring out where overlay ismost sensitive to an asymmetry parameter q. The gradient of OV withrespect to asymmetry parameter q is expressed as:

$\begin{matrix}{\frac{\partial{OV}}{\partial q} = {\left( {{\frac{1}{K}\frac{\partial A}{\partial q}} - {\frac{A}{K^{2}}\frac{\partial K}{\partial q}}} \right) \approx {\frac{1}{K}\frac{\partial A}{\partial q}}}} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

At a pixel, intensity can be generically expressed as I_(x) (with xalternatively being p_pb, m_pb, p_nb and m_nb):I _(x) =∫dσl _(s)(σ)P _(align)(σ)P _(obj)(σ)∫dρl _(RCWA)(σ)ε(σ−ρ)   (Eq.6)

Here, σ indicates the coordinate of the illumination pupil, and, ρindicates convolution coordinate of the convolution kernel, ε. Theconvolution kernel is a function related to the finite spot of themetrology or to the finite size of the metrology mark. P_(align)indicates the pupil of the alignment camera and P_(obj) indicates thepupil filter in the exit pupil of the objective lens. Is indicatesintensity of the illumination (the parameter to be optimized for minimumsensitivity to process asymmetry or for maximum K). I_(RCWA) indicatesthe intensity as calculated by rigorous coupled wave analysis (RCWA),which is a semi-analytic method in computational electromagneticsapplied to solve diffraction scattering from a metrology target with aperiodic structure, such as described here. The gradient of intensitywith respect to the asymmetry parameter q is expressed as:

$\begin{matrix}{\frac{\partial I_{x}}{\partial q} = {\int{d\;\overset{\_}{\sigma}\;{I_{s}\left( \overset{\_}{\sigma} \right)}{P_{align}\left( \overset{\_}{\sigma} \right)}{P_{obj}\left( \overset{\_}{\sigma} \right)}{\int{d\overset{\_}{\rho}\frac{\partial{I_{RCWA}\left( \overset{\_}{\sigma} \right)}}{\partial q}{K\left( {\overset{\_}{\sigma} - \overset{\_}{\rho}} \right)}}}}}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

The gradient of intensity from RCWA is independent of illuminationintensity, I_(s). So it can be pre-calculated for fast optimization. Tooptimize illumination to minimize overlay sensitivity to the parameterq, the following second order derivative equation is used to expressgradient with respect to illumination:

$\begin{matrix}{\frac{\partial^{2}{OV}}{{\partial I_{s}}{\partial q}} = \left( {{{- \frac{1}{K^{2}}}\frac{\partial K}{\partial I_{s}}\frac{\partial A}{\partial q}} + {\frac{1}{K}\frac{\partial^{2}A}{{\partial q}{\partial I_{s}}}} - {\frac{1}{K^{2}}\frac{\partial A}{\partial I_{s}}\frac{\partial K}{\partial q}} + {2\frac{A}{K^{3}}\frac{\partial K}{\partial I_{s}}\frac{\partial K}{\partial q}} - {\frac{A}{K^{2}}\frac{\partial^{2}K}{{\partial q}{\partial I_{s}}}}} \right)} & \left( {{Eq}.\mspace{14mu} 8} \right)\end{matrix}$

Using the full equation above for

$\frac{\partial^{2}{OV}}{{\partial I_{s}}{\partial q}}$does not result in more computational time, compared to a case where itis approximated that

${\frac{\partial^{2}{OV}}{{\partial I_{s}}{\partial q}} \approx {\frac{1}{K}\frac{\partial^{2}A}{{\partial I_{s}}{\partial q}}}}.$

The next step is to define a cost function (CF) for the illumination.The CF is the mathematical function to be optimized to get the optimizedillumination. The cost function has the asymmetry parameter q as avariable. The goal of the optimization problem is to minimize the costfunction, which is expressed as the following equation:

$\begin{matrix}{{\min\mspace{14mu}{{CF}\left( I_{s} \right)}} = {\sum_{i = 1}^{m}\left\lbrack \frac{{\partial O}{V(\alpha)}}{\partial q_{i}} \right\rbrack^{n}}} & \left( {{Eq}.\mspace{14mu} 9} \right)\end{matrix}$

${\frac{K(\alpha)}{D{E(\alpha)}}} > \beta$is the constraint used in the minimization equation above, where β is afixed (non-floating) parameter, and a is a floating parameter. In theequation above, the norm (indicated as n in the equation) of the overlaysensitivity

$\frac{{\partial O}V}{\partial q}$is minimized. The number of parameters to optimize the illumination overis m (i.e., i=1, 2, . . . , m). Examples of the asymmetry parameters areasymmetry in side wall angle (aSWA), floor tilt, etch depth etc. FIGS.6A-B show a target structure with a top grating layer G1 and a bottomgrating layer G2, where structural asymmetries are shown in anexaggerated manner for illustrative purposes. FIG. 6A schematicallyshows that there is a difference between side wall angle for the wall601 and side wall angle of the wall 602. The side wall angle difference(ΔSWA) gives rise to the aSWA parameter. FIG. 6B schematically shows thefloor 610 is tilted in the bottom grating layer. This asymmetry givesrise to the floor tilt parameter. For the DRAM SN layer device shown inFIG. 5, the overlay sensitivity due to process variation is studied forthe bottom tungsten layer 514 for three illumination polarizations:transverse electric (TE), transverse magnetic (TM) and a combination ofboth polarizations (BP). The results are tabulated in Table I in FIG. 7.Persons skilled in the art would appreciate that in addition topolarization, wavelength of the illumination source may also be changed,though not specifically described in the example embodiments here.

In the cost function minimization problem, α is the polarization mixingparameter to take advantage of the sensitivity due to TE and TMpolarization, according to the following equation:I _(RCWA) =αI _(TE) _(RCWA) +(1−α)I _(TM) _(RCWA)   (Eq. 10)

In the above equation, α=0.5 for BP, α=1 for TE polarization, and α=0for TM polarization. α can also be optimized to minimize sensitivity toprocess asymmetry. The optimization constraint is recast as a barrierfunction expressed generically as x^(−b). The barrier function isrelated to stack sensitivity (SS) of a particular device structure. Abarrier function is used in optimization to represent a less than (orgreater than) Heaviside function constraint. The barrier function has acontinuous gradient while a Heaviside function does not. For example, ina standard Source-Mask Optimization (SMO) program Tachyon, offered byASML, the barrier function uses a hard constraint with x⁻³, i.e., b=3.In general, the barrier function is expressed as:

$\begin{matrix}{{B\left( I_{s} \right)} = {- {\mu\left\lbrack {\beta - {\frac{K(\alpha)}{D{E(\alpha)}}}} \right\rbrack}^{- b}}} & \left( {{Eq}.\mspace{14mu} 11} \right)\end{matrix}$

Here, μ is a fixed parameter which is related to the weight of thebarrier function in Eq. 11 compared to the unconstrained minimization inEq. 9. If m is large, the barrier function has less weight (because ofthe minus sign) than the unconstrained minimization. In a non-limitingexample, values of α, μ and b may be equal to 0.5, 1 and 1 respectively.

The following equation represents an auxiliary cost function to beminimized, which is the original cost function in Eq. 9 modified by thebarrier function in Eq. 11:

$\begin{matrix}{{\min\mspace{14mu}{{CF}\left( I_{s} \right)}} = {{\sum_{i = 1}^{m}\left\lbrack \frac{{\partial O}{V(\alpha)}}{\partial q_{i}} \right\rbrack^{n}} - {\mu\left\lbrack {\beta - {\frac{K(\alpha)}{D{E(\alpha)}}}} \right\rbrack}^{- b}}} & \left( {{Eq}.\mspace{14mu} 12} \right)\end{matrix}$

For minimizing the auxiliary cost function, the gradient of theauxiliary cost function needs to be zero.

$\begin{matrix}{{\frac{\partial}{\partial I_{s}}{{CF}\left( I_{s} \right)}} = {{n{\sum_{i = 1}^{m}{\left\lbrack \frac{{\partial O}{V(\alpha)}}{\partial q_{i}} \right\rbrack^{n - 1}\frac{{\partial^{2}O}{V(\alpha)}}{{\partial I_{s}}{\partial q_{i}}}}}} + {b{\mu\left\lbrack {\beta - {\frac{K(\alpha)}{D{E(\alpha)}}}} \right\rbrack}^{{- b} - 1}\frac{\partial}{\partial I_{s}}{\frac{K(\alpha)}{D{E(\alpha)}}}}}} & \left( {{Eq}.\mspace{14mu} 13} \right)\end{matrix}$

Since out of the three process variations studied (etch depth, aSWA andfloor tilt), there was no overlay sensitivity observed for etch depth(as seen in the results Table in FIG. 7), the root mean square (rms)asymmetry due to process variation is simplified in terms of the twoasymmetry parameters aSWA and floor tilt:

$\begin{matrix}{\sqrt{CF} = \sqrt{{\frac{{\partial O}V}{\partial{SWA}}}^{2} + {\frac{{\partial O}V}{\partial{FloorTilt}}}^{2}}} & \left( {{Eq}.\mspace{14mu} 14} \right)\end{matrix}$

CF is either the unconstrained cost function in Eq. 9 or the auxiliarycost function of Eq. 11, constrained by the barrier function.Simulations have shown that the value of asymmetry CF is reduced from1.56 for an initial configuration of the illumination to 1.22 for anillumination optimized for structural asymmetry, i.e. a 12% improvementis achieved by the embodiments of the present disclosure. The percentageimprovement varies depending on the target structure, but at least a5-50% improvement is seen routinely in the experimental results. FIG. 8shows a plot of final illumination configuration for the device DRAM SNlayer structure shown in FIG. 5, when an unconstrained cost function CFis minimized using the algorithm of the present disclosure. FIG. 9 showsthe spatial distribution of the barrier function, B, where stacksensitivity SS>0.1. FIG. 10 shows the final illumination configurationafter the final iteration of the optimization algorithm.

FIG. 11 shows a plot of overlay sensitivity with respect to theasymmetry parameter aSWA, showing that as the number of iterationsincreases, and correspondingly the illumination shape changes, thesensitivity to aSWA decreases.

In FIG. 12, the first order derivative function (1/K)*∂A/∂aSWA isplotted vs. the number of iterations, and the results are shown by thecircles. FIG. 12 also shows the second order derivative(A/K²)*∂|A|/∂aSWA, plotted as the triangles. Since the value of thesecond order derivative (A/K²)*∂|A|/∂aSWA is nearly zero, i.e. thegradient of the first order derivative doesn't change with the number ofiterations, mathematically, overlay sensitivity ∂OV/∂aSWA can beapproximated by only the first order derivative, (1/K)*∂|A|/∂aSWA.

FIG. 13 plots the value of K vs. the number of iterations, and FIG. 14plots ∂|A|/∂aSWA vs. the number of iterations. Illumination pixels areadjusted so that asymmetry A decreases. K decreases with number ofiterations, and ∂|A|/∂aSWA decreases faster than the increase of 1/K.Consequently, overlay sensitivity ∂OV|/∂aSWA decreases. The resultsprove that by adjusting illumination shape pixel by pixel, sensitivityof overlay measurement to process-induced structural asymmetry can bereduced.

Some of the features and advantages of the present optimizationtechniques are, among other things: removal of zeroth order light can bequickly done through simple geometry constraints; and, most of thedevice structures are not restricted by minimum stack sensitivitybarrier function, B. In one embodiment, illumination optimization isbased on maximization of K rather than reducing A in one embodiment. Theasymmetry response, A, is independent of angle in the alignment camera.In an alternative embodiment, illumination optimization may be based ona reduction in sensitivity in A rather than an increase in K.Furthermore, in an embodiment, the response in the pupil plane may beexamined in addition to the image plane, because the pupil plane hasmore information on asymmetry.

FIG. 15 schematically shows a flow for a method according to anembodiment. In 1510, the intensity distribution in a pupil plane of themetrology apparatus is spatially divided into a plurality of pixels. In1520, an effect of a structural asymmetry in a target on a measurementby the metrology apparatus on the target is reduced, by adjusting, usinga computer, intensities of the plurality of pixels. The pupil plane maybe an illumination pupil or a detection pupil. The measurement maymeasure overlay, focus, aberration or a combination thereof. Thestructural asymmetry may include one or more of: a difference in sidewall angle (SWA), floor tilt, and etch depth. Adjusting the intensitiesmay include computing a cost function that represents the effect andthat is a function of the intensities. The cost function may representcontrast of an image of the target. Adjusting the intensities mayinclude finding values of the intensities that locally or globallyminimizes or maximizes the cost function. The cost function may beconstrained. In optional 1530, one or more pixels from the plurality ofpixels are identified, where the one or more pixels do not contribute toa signal used by the metrology apparatus in the measurement, or wherethe contribution of the one or more pixels to the signal is below athreshold. In optional 1540, the intensities at the one or moreidentified pixels are adjusted. The measurement may be an overlaymeasurement and the one or more pixels do not contribute to adiffraction order used in the overlay measurement. The diffraction ordermay be a +1^(st) diffraction order or −1^(st) diffraction order.Reducing the effect of the structural asymmetry may include adjustingpolarizations at the pixels. Reducing the effect of the structuralasymmetry may include adjusting bandwidths at the pixels. Reducing theeffect of the structural asymmetry may include adjusting wavelengths atthe pixels.

FIG. 16 schematically shows a flow for a method according to anembodiment. In 1610, a parameter of the metrology apparatus or of ameasurement by the metrology apparatus on a target is adjusted, based ona characteristic of the target. The measurement may be selected from agroup consisting of a measurement of overlay, a measurement of focus, ameasurement of aberration, and a combination thereof. The parameter maybe selected from a group consisting of an intensity at an illuminationpupil of the metrology apparatus, a polarization at an illuminationpupil of the metrology apparatus, a wavelength at an illumination pupilof the metrology apparatus, a bandwidth at an illumination pupil of themetrology apparatus, an intensity at a detection pupil of the metrologyapparatus, a polarization at a detection pupil of the metrologyapparatus, a wavelength at a detection pupil of the metrology apparatus,a bandwidth at a detection pupil of the metrology apparatus, and acombination thereof. For example, the intensity at the detection pupilor the source pupil may be adjusted using an aperture on the opticalpath. The parameter may be a characteristic of projection optics of themetrology apparatus or a characteristic of a source of the metrologyapparatus. Adjusting the parameter may impact a quality of themeasurement. The quality may be detectability of the target, accuracy ofthe measurement, or robustness of the measurement. Adjusting theparameter may include computing a cost function that represents thequality and is a function of the parameter. The cost function mayrepresent contrast of an image of the target. Adjusting the parametermay include finding a value of the parameter that locally or globallyminimizes or maximizes the cost function. The cost function isconstrained. Adjusting the parameter may include iteratively computingthe cost function and adjusting the parameter until a terminationcriterion is met. The cost function may be a function of thecharacteristic of the target. In optional 1620, the characteristic ofthe target is adjusted. In optional 1630, the adjusted parameterassociated with the characteristic of the target is stored. Thecharacteristic of the target may include a location of the target on asubstrate.

The invention may further be described using the following clauses:

-   1. A method of adjusting a metrology apparatus, the method    comprising:    -   spatially dividing an intensity distribution in a pupil plane of        the metrology apparatus into a plurality of pixels; and    -   reducing an effect of a structural asymmetry in a target on a        measurement by the metrology apparatus on the target, by        adjusting, using a computer, intensities of the plurality of        pixels.-   2. The method of clause 1, wherein the pupil plane is an    illumination pupil or a detection pupil.-   3. The method of clause 1 or clause 2, wherein the measurement    measures overlay, focus, aberration or a combination thereof.-   4. The method of any of clauses 1 to 3, wherein the structural    asymmetry comprises one or more of: a difference in side wall angle    (SWA), floor tilt, and etch depth.-   5. The method of any of clauses 1 to 4, wherein adjusting the    intensities comprises computing a cost function that represents the    effect and that is a function of the intensities.-   6. The method of clause 5, wherein the cost function represents    contrast of an image of the target.-   7. The method of clause 5, wherein adjusting the intensities further    comprises finding values of the intensities that locally or globally    minimizes or maximizes the cost function.-   8. The method of clause 5, wherein the cost function is constrained.-   9. The method of any of clauses 1 to 8, further comprising:    identifying one or more pixels from the plurality of pixels, wherein    the one or more pixels do not contribute to a signal used by the    metrology apparatus in the measurement or wherein the contribution    of the one or more pixels to the signal is below a threshold, and    adjusting the intensities at the one or more identified pixels.-   10. The method of clause 9, wherein the measurement is an overlay    measurement and the one or more pixels do not contribute to a    diffraction order used in the overlay measurement.-   11. The method of clause 10, wherein the diffraction order is a    +1^(st) diffraction order or −1^(st) diffraction order.-   12. The method of any of clauses 1 to 11, wherein reducing the    effect of the structural asymmetry further comprises adjusting    polarizations at the pixels.-   13. The method of any of clauses 1 to 12, wherein reducing the    effect of the structural asymmetry further comprises adjusting    bandwidths at the pixels.-   14. The method of any of clauses 1 to 13, wherein reducing the    effect of the structural asymmetry further comprises adjusting    wavelengths at the pixels.-   15. A computer program product comprising a non-transitory    computer-readable medium having instructions thereon, the    instructions when executed by a computer implementing a method    comprising:-   spatially dividing an intensity distribution in a pupil plane of the    metrology apparatus into a plurality of pixels;-   reducing an effect of a structural asymmetry in a target on a    measurement by the metrology apparatus on the target, by adjusting,    using a computer, intensities of the plurality of pixels.-   16. The computer program product of clause 15, wherein the pupil    plane is an illumination pupil or a detection pupil.-   17. The computer program product of any of clauses 15 to 16, wherein    the measurement measures overlay, focus, aberration or a combination    thereof.-   18. The computer program product of any of clauses 15 to 17, wherein    the structural asymmetry comprises one or more of: a difference in    side wall angle (SWA), floor tilt, and etch depth.-   19. The computer program product of any of clauses 15 to 18 wherein    adjusting the intensities comprises computing a cost function that    represents the effect and is a function of the intensities.-   20. The computer program product of clause 19, wherein the cost    function represents contrast of an image of the target.-   21. The computer program product of clause 19, wherein adjusting the    intensities further comprises finding values of the intensities that    locally or globally minimizes or maximizes the cost function.-   22. The computer program product of clause 19, wherein the cost    function is constrained.-   23. The computer program product of any of clauses 15 to 22, further    comprising:-   identifying one or more pixels from the plurality of pixels, wherein    the one or more pixels do not contribute to a signal used by the    metrology apparatus in the measurement or wherein the contribution    of the one or more pixels to the signal is below a threshold, and    adjusting the intensities at the one or more identified pixels.-   24. The computer program product of clause 23, wherein the    measurement is an overlay measurement and the one or more pixels do    not contribute to a diffraction order used in the overlay    measurement.-   25. The computer program product of clause 24, wherein the    diffraction order is a +1^(st) diffraction order or −1^(st)    diffraction order.-   26. The computer program product of any of clauses 15 to 25, wherein    reducing the effect of the structural asymmetry further comprises    adjusting polarizations at the pixels.-   27. The computer program product of any of clauses 15 to 26, wherein    reducing the effect of the structural asymmetry further comprises    adjusting bandwidths at the pixels.-   28. The computer program product of any of clauses 15 to 27, wherein    reducing the effect of the structural asymmetry further comprises    adjusting wavelengths at the pixels.-   29. A method of adjusting a metrology apparatus, the method    comprising: adjusting a parameter of the metrology apparatus or of a    measurement by the metrology apparatus on a target, based on a    characteristic of the target.-   30. The method of clause 29, wherein the measurement is selected    from a group consisting of a measurement of overlay, a measurement    of focus, a measurement of aberration, and a combination thereof.-   31. The method of any of clauses 29 to 30, wherein the parameter is    selected from a group consisting of an intensity at an illumination    pupil of the metrology apparatus, a polarization at an illumination    pupil of the metrology apparatus, a wavelength at an illumination    pupil of the metrology apparatus, a bandwidth at an illumination    pupil of the metrology apparatus, an intensity at a detection pupil    of the metrology apparatus, a polarization at a detection pupil of    the metrology apparatus, a wavelength at a detection pupil of the    metrology apparatus, a bandwidth at a detection pupil of the    metrology apparatus, and a combination thereof.-   32. The method of any of clauses 29 to 31, wherein the parameter is    a characteristic of projection optics of the metrology apparatus or    a characteristic of a source of the metrology apparatus.-   33. The method of any of clauses 29 to 32, wherein adjusting the    parameter impacts a quality of the measurement.-   34. The method clause 33, wherein the quality is detectability of    the target, accuracy of the measurement, or robustness of the    measurement.-   35. The method of clause 33, wherein adjusting the parameter    comprises computing a cost function that represents the quality and    is a function of the parameter.-   36. The method of clause 35, wherein the cost function represents    contrast of an image of the target.-   37. The method of clause 35, wherein adjusting the parameter further    comprises finding a value of the parameter that locally or globally    minimizes or maximizes the cost function.-   38. The method of clause 35, wherein the cost function is    constrained.-   39. The method of clause 35, wherein adjusting the parameter    comprises iteratively computing the cost function and adjusting the    parameter until a termination criterion is met.-   40. The method of clause 35, wherein the cost function is a function    of the characteristic of the target.-   41. The method of any of clauses 29 to 40, further comprising    adjusting the characteristic of the target.-   42. The method of any of clauses 29 to 41, further comprising    storing the adjusted parameter associated with the characteristic of    the target.-   43. The method of any of clauses 29 to 42, wherein the    characteristic of the target comprises a location of the target on a    substrate.-   44. A method comprising:-   setting a parameter of a metrology apparatus or of a measurement by    the metrology apparatus on a target to a value adjusted based on a    characteristic of the target;-   measuring the target with the metrology apparatus.-   45. A method comprising:-   setting a characteristic of a target to a value based on which a    parameter of a metrology apparatus or of a measurement by the    metrology apparatus on the target is adjusted;-   fabricating the target on a substrate.-   46. A computer program product comprising a non-transitory    computer-readable medium having instructions thereon, the    instructions when executed by a computer implementing a method    comprising:-   adjusting a parameter of the metrology apparatus or of a measurement    by the metrology apparatus on a target, based on a characteristic of    the target.-   47. The computer program product of clause 46, wherein the    measurement is selected from a group consisting of a measurement of    overlay, a measurement of focus, a measurement of aberration, and a    combination thereof.-   48. The computer program product of any of clauses 46 to 47, wherein    the parameter is selected from a group consisting of an intensity at    an illumination pupil of the metrology apparatus, a polarization at    an illumination pupil of the metrology apparatus, a wavelength at an    illumination pupil of the metrology apparatus, a bandwidth at an    illumination pupil of the metrology apparatus, an intensity at a    detection pupil of the metrology apparatus, a polarization at a    detection pupil of the metrology apparatus, a wavelength at a    detection pupil of the metrology apparatus, a bandwidth at a    detection pupil of the metrology apparatus, and a combination    thereof.-   49. The computer program product of any of clauses 46 to 48, wherein    the parameter is a characteristic of projection optics of the    metrology apparatus or a characteristic of a source of the metrology    apparatus.-   50. The computer program product of any of clauses 46 to 49, wherein    adjusting the parameter impacts a quality of the measurement.-   51. The computer program product clause 50, wherein the quality is    detectability of the target, accuracy of the measurement, or    robustness of the measurement.-   52. The computer program product of clause 50, wherein adjusting the    parameter comprises computing a cost function that represents the    quality and is a function of the parameter.-   53. The computer program product of clause 50, wherein the cost    function represents contrast of an image of the target.-   54. The computer program product of clause 50, wherein adjusting the    parameter further comprises finding a value of the parameter that    locally or globally minimizes or maximizes the cost function.-   55. The computer program product of clause 50, wherein the cost    function is constrained.-   56. The computer program product of clause 50, wherein adjusting the    parameter comprises iteratively computing the cost function and    adjusting the parameter until a termination criterion is met.-   57. The computer program product of clause 50, wherein the cost    function is a function of the characteristic of the target.-   58. The computer program product of any of clauses 46 to 57, wherein    the method further comprises adjusting the characteristic of the    target.-   59. The computer program product of any of clauses 46 to 58, wherein    the method further comprises storing the adjusted parameter    associated with the characteristic of the target.-   60. The computer program product of any of clauses 46 to 59, wherein    the characteristic of the target comprises a location of the target    on a substrate.-   62. A computer program product comprising a non-transitory    computer-readable medium having instructions thereon, the    instructions when executed by a computer implementing a method    comprising:-   setting a parameter of a metrology apparatus or of a measurement by    the metrology apparatus on a target to a value adjusted based on a    characteristic of the target; measuring the target with the    metrology apparatus.-   63. A computer program product comprising a non-transitory    computer-readable medium having instructions thereon, the    instructions when executed by a computer implementing a method    comprising:-   setting a characteristic of a target to a value based on which a    parameter of a metrology apparatus or of a measurement by the    metrology apparatus on the target is adjusted; fabricating the    target on a substrate.

Persons skilled in the art would appreciate that additional steps can beadded in the process flow, and/or some steps shown in the flowchart canbe substituted by steps specific to a certain lithography processwithout departing from the scope of the disclosure. The descriptionsabove are intended to be illustrative, not limiting. Thus, it will beapparent to one skilled in the art that modifications may be made to theembodiments as described without departing from the scope of the claimsset out below.

The invention claimed is:
 1. A method of adjusting a metrologyapparatus, the method comprising: setting or adjusting a parameter ofthe metrology apparatus for measuring a target or of a measurement bythe metrology apparatus of the target, to a value based on acharacteristic of the target, the characteristic comprising a structuralasymmetry within the target or a value representing a location of thetarget on a substrate.
 2. The method of claim 1, wherein the measurementis one or more selected from: a measurement of overlay, a measurement offocus, and/or a measurement of aberration.
 3. The method of claim 1,wherein the parameter is one or more selected from: an intensity at anillumination pupil of the metrology apparatus, a polarization at anillumination pupil of the metrology apparatus, a wavelength at anillumination pupil of the metrology apparatus, a bandwidth at anillumination pupil of the metrology apparatus, an intensity at adetection pupil of the metrology apparatus, a polarization at adetection pupil of the metrology apparatus, a wavelength at a detectionpupil of the metrology apparatus, and/or a bandwidth at a detectionpupil of the metrology apparatus.
 4. The method of claim 1, wherein theparameter is a characteristic of projection optics of the metrologyapparatus and/or a characteristic of radiation illumination of themetrology apparatus.
 5. The method of claim 1, wherein the setting oradjusting the parameter impacts a quality of the measurement of thetarget.
 6. The method of claim 5, wherein the quality representsdetectability of the target, accuracy of the measurement, and/orrobustness of the measurement.
 7. The method of clause 20, wherein thesetting or adjusting the parameter comprises computing a cost functionthat represents the quality and is a function of the parameter.
 8. Themethod of claim 7, wherein the cost function represents contrast of animage of the target.
 9. The method of claim 7, wherein the setting oradjusting the parameter further comprises finding a value of theparameter that locally or globally minimizes or maximizes the costfunction.
 10. The method of claim 7, wherein the cost function isconstrained.
 11. The method of claim 7, wherein the cost function is afunction of the characteristic of the target.
 12. The method of claim 1,further comprising adjusting the characteristic of the target.
 13. Themethod of claim 1, wherein the characteristic of the target comprises avalue representing a location of the target on a substrate.
 14. Themethod of claim 1, wherein the characteristic of the target comprises astructural asymmetry within the target.
 15. A computer program productcomprising a non-transitory computer-readable medium having instructionstherein, the instructions, upon execution by a computer system,configured to cause the computer system to at least: set or adjust aparameter of a metrology apparatus for measuring a target or of ameasurement by the metrology apparatus of the target, to a value basedon a characteristic of the target, the characteristic comprising astructural asymmetry within the target or a value representing alocation of the target on a substrate.
 16. The computer program productof claim 15, wherein the parameter is one or more selected from: anintensity at an illumination pupil of the metrology apparatus, apolarization at an illumination pupil of the metrology apparatus, awavelength at an illumination pupil of the metrology apparatus, abandwidth at an illumination pupil of the metrology apparatus, anintensity at a detection pupil of the metrology apparatus, apolarization at a detection pupil of the metrology apparatus, awavelength at a detection pupil of the metrology apparatus, and/or abandwidth at a detection pupil of the metrology apparatus.
 17. Thecomputer program product of claim 15, wherein the setting or adjustingof the parameter impacts a quality of the measurement of the target. 18.The computer program product of claim 17, wherein the instructionsconfigured to cause the computer system to set or adjust the parameterare further configured to compute a cost function that represents thequality and is a function of the parameter.
 19. The computer programproduct of claim 15, wherein the characteristic of the target comprisesa value representing a location of the target on a substrate.
 20. Thecomputer program product of claim 15, wherein the characteristic of thetarget comprises a structural asymmetry within the target.
 21. A methodcomprising: setting or adjusting a parameter of a metrology apparatusfor measuring a target or of a measurement by the metrology apparatus ofthe target, to a value set or adjusted based on a characteristic of thetarget, the characteristic comprising a structural asymmetry within thetarget; and measuring the target with the metrology apparatus.